1. Field of the Invention
The present invention relates to a sealed-in foil and associated lamp containing the sealed-in foil. The present invention relates in particular to molybdenum foils which are used in press seals, such as are standard for sealing incandescent lamps and discharge lamps.
2. Background Information
U.S. Pat. No. 5,021,711 describes a sealed-in foil and associated lamp containing the foil. In order to be better protected from oxidation, the foil is provided with a protective layer of Al, Cr, Si, Ti or Ta. The thickness of the foil ranges from 5 to 100 nm.
A similar technique is described in German Patent A 30 06 846, in which layers of Ta, Nb, V, Cr, Ti, Y, La, Hf or Sc are used for the same purpose. The layer thickness ranges from 10 to 200 nm.
In practice, partial chrome plating is usually used to protect the molybdenum foils from oxidation in the region of the welded connection between the foil and the pin. In this extremely labor-intensive method, the welded connections made between the pin and the foil by resistance welding are manually forced into a sand-like medium to the height up to which chrome plating is to be applied. Partial chromium deposition by chemical reactions takes place in a process which is not very environmentally sound. Improved thermal stability of the foil-pin connections is achieved by this chromium deposition (oxidation protection). Thus thermal stability up to about 550xc2x0 C. is possible.
In some lamps it is not oxidation of the foil-pin connections that is responsible for failure of the foil seal, but attack on the molybdenum foil by the corrosive fill constituents (such as metal halides) or even fill gases. Heretofore this corrosion has been limited by sand-blasting the molybdenum foil, thus leading to an improvement of the glass-to-metal joint. Sand blasting leads to high reject rates in resistance welding, however, since thereby nonconductive Al2O3 particles remain on the Mo foil surface. Moreover, the wear of the resistance welding. electrodes increases toga great degree. In sand-blasted foils, the electrodes have to be replaced after about 70 welding operations (compared with a replacement interval of about 1000 weld operations for untreated foil), and so the electrodes have to be changed frequently.
The object of the present invention is to provide a sealed-in foil which is well protected from oxidation and corrosion and. in which weldability is nevertheless assured to the greatest extent possible. This object is achieved by the present invention.
To prevent-oxidation and corrosion, and to achieve good weldability, according to the present invention, a molybdenum foil is coated partly or preferably all over with pure ruthenium or a compound or alloy containing ruthenium. Pure ruthenium in particular, as well as a molybdenum-ruthenium alloy of a eutectic composition, is suitable as the coating material.
The present invention is thus directed to a sealed-in foil for a lamp comprising a metal base member of pure molybdenum or doped molybdenum, and a coating comprising a ruthenium-containing layer applied on at least a portion of the metal base member.
The present invention also concerns a lamp containing such sealed-in foil.
The present invention further relates to a lamp comprising a lamp vessel made of a hard glass or quartz glass, the lamp vessel enclosing a discharge space, a luminous agent contained within the discharge space of the lamp vessel, an interior current lead, one end of which passes into the discharge space, and a seal provided at an end of the lamp vessel, the seal comprising a gas-tight feedthrough which surrounds a portion of the interior current lead, the gas-tight feedthrough comprising a molybdenum foil, at least a portion of the molybdenum foil being provided with a ruthenium-containing layer made of a ruthenium-containing material.
The present invention further concerns a process for making an electrically conductive-connection between a molybdenum foil and a metal wire comprising providing a molybdenum foil having at least a portion thereof with a ruthenium-containing coating and connecting the metal wire to the foil in a region containing the coating.